[Integrated] RFR: git-webrev: use HEAD when branch has no commits

[Erik Helin]

Changeset: d0eb7970
Author:    Erik Helin <ehelin at openjdk.org>
Date:      2020-03-11 17:00:12 +0000
URL:       https://git.openjdk.java.net/skara/commit/d0eb7970

git-webrev: use HEAD when branch has no commits

Reviewed-by: rwestberg

! cli/src/main/java/org/openjdk/skara/cli/GitWebrev.java